To restart your device, disconnect the power cord from the back of the device or from the power outlet, and then plug it back in. Make sure that you are using the power supply provided with your Fire TV device. Connect the included power adapter into the power port on the back of your Fire TV device, then plug the other end into a power outlet.
You might want to try any of the following as well:
If you are using an HDMI hub, disconnect your Fire TV device from the hub and try plugging it directly into the TV.
Try switching to a different HDMI port on your television. Remember which HDMI port you used to connect your Fire TV device and switch to the corresponding input channel on your TV.
Please let me know if any of the above resolves your issue.